This invention relates generally to lubrication and lubricants, and more particularly to a hybrid lubricant in which solid lubricant particles are dispersed in a fluid lubricant carrier that may include a small but effective amount of halocarbon oil to react with the surface being lubricated.
Even the most carefully finished metal surfaces have minute projections and depressions therein which introduce resistance when one surface shifts relative to another. The application of a fluid lubricant to these surfaces reduces friction by interposing a film of oil therebetween, this being known as hydrodynamic lubrication. In a bearing, for example, the rotation of the journal causes oil to be drawn between it and the bearing so that the two metal surfaces are then separated by a very thin oil film. The degree of bearing friction depends on the viscosity of the oil, the speed of rotation and the load on the journal.
Should the journal start its rotation after a period of rest, it may not drag enough oil to float the surfaces apart; hence friction would then be considerably greater, the friction being independent of the viscosity of the lubricant and being related only to the load and to the "oiliness" property of the residual lubricant to stick tightly to the metal surfaces. This condition is referred to as "boundary lubrication," for then the moving parts are separated by a film of only molecular thickness. This may cause serious damage to overheated bearing surfaces.
The two most significant characteristics of a hydrodynamic lubricant are its viscosity and its viscosity index, the latter being the relationship between viscosity and temperature. The higher the index, the less viscosity will change with temperature. Fluid lubricants act not only to reduce friction, but also to remove heat developed within the machinery and as a protection against corrosion.
Though fluid film separation of rubbing surfaces is the most desirable objective of lubrication, it is often unobtainable in practice. Thus bearings built for full fluid lubrication during most of their operating phases actually experience solid-to-solid contact when starting and stopping. Solid surfaces in rubbing contact are characterized by coefficients of friction varying between 0.04 (Teflon on steel) and &gt; 100 (pure metals in vacuo). In contrast to fluid lubrication, solid lubrication is usually accompanied by wear of rubbing parts. Optical inspection of the surfaces after rubbing invariably reveals microscopic damage of the metal both when unlubricated and lubricated.
Typical solid lubricants are soft metals such as lead, the layer lattice crystals such as graphite and molybdenum disulphide, as well as the crystalline polymers such as Teflon (polytetrafluoroethylene). The integral bonding of these solid lubricants to the surfaces of the bodies to be lubricated is essential for good performance.
Under the severe operating conditions unusually encountered in automotive transmissions and in internal combustion engines, hydrodynamic or fluid lubrication is inadequate to minimize friction and wear; for fluid film separation of the rubbing surfaces is not possible throughout all phases of operation. Hence, the ideal lubricant for an engine or other mechanism having moving parts is one which combines hydrodynamic with solid lubrication. In this way, when adequate separation exists between the rubbing surfaces, a protective fluid film is interposed therebetween; and when the surfaces are in physical contact with each other, friction therebetween is minimized by layers of solid lubricant bonded to the surfaces.
In theory, one can best approach this ideal by lining the rubbing parts of engines with solid lubricant layers which are integrally bonded thereto, concurrent use being made of a lubricating oil which functions not only to provide hydrodynamic lubrication but also to cool the rubbing parts. In addition, the oil may carry synthetic organic chemicals to perform other functions to counteract wear and prevent corrosion.
The practical difficulty with attaining this ideal is that the parts coated with solid lubricants, such as a TFE layer, are very expensive and therefore add considerably to the overall cost of the engine. Moreover, in TFE-coated parts which operate under rigorous conditions, the solid lubricant layers bonded thereto have a relatively short working life, so that it is not long before the only lubricant which remains effective in the engine is the fluid lubricant.
In order to provide a lubricating action which is both solid and fluid, my prior U.S. Pat. No. 3,933,656 discloses a modified oil lubricant which is suitable for an internal combustion engine provided with an oil filter as well as for many other applications which call for effective lubrication throughout all phases of operation. This modified lubricant is constituted by major amounts of a conventional lubricating oil intermingled with minor amounts of an aqueous dispersion of polytetrafluoroethylene particles in the sub-micronic range in combination with a neutralizing agent which stabilizes the dispersion to prevent agglomeration and coagulation of the particles. Thus the modified lubricant is capable of passing through the oil filter without separating the solid particles from the oil in which it is dispersed.
As pointed out in my prior patent, when use is made of this modified lubricant in an internal combustion engine, the engine "runs progressively smoother as the internal surfaces acquire a coating of Teflon." Thus the Teflon solid lubricant coating is applied to the rubbing parts by the circulating fluid lubricant. This modified lubricant has many significant advantages; for, as indicated in my prior patent, it reduces wear and thereby prolongs engine life, it makes possible a sharp reduction in the emission of pollutants and also effects a significant improvement in fuel economy, the last factor being of overriding importance in a fuel-short world.
In the modified lubricant disclosed in my prior patent, a stabilized aqueous dispersion of solid lubricant particles (PTFE) is intermingled with the oil lubricant in the engine itself. Because of the water involved, the aqueous dispersion tends, when introduced into the oil, to break up into rather large globules, rather than to become evenly dispersed or homogenized in the oil. Hence, my modified lubricant, though effective in reducing friction, is not as effective as it would be with a more uniform dispersion.
Moreover, the Teflon coatings which form on the surface of the internal rubbing metal parts do not always remain securely bonded thereto in all areas, and while the solid lubricant coatings on some areas are often renewed in the course of engine operation, this factor also militates against the full and effective utilization of the modified lubricants disclosed in my prior patent.